FIELD OF THE INVENTION
This invention relates to a method of preparing a poly-crystalline silicon film and, more particularly, a method of preparing a poly-crystalline silicon film characterized in dehydrogenation treatment of an amorphous silicon to be deposited by a plasma chemical vapor deposition process.
Amorphous silicon thin film transistors (a-Si TFTS) have been used for conventional liquid crystal display devices as pixel switching elements. The a-Si TFT includes an active layer made of the a-Si film. This is because the a-Si film can be uniformly deposited at a relatively lower temperature on a glass substrate by a plasma enhanced chemical vapor deposition (PECVD) process. The glass substrate (lower heat-resistance) is suitable for making a display region larger in area.
Driver-circuit integrated type liquid crystal display devices, on the other hand, have been recently developed. In this type liquid crystal display device, driver circuits are disposed at the surrounding of the display region (pixel region) on the same substrate as the pixel switching elements. TFTs are also used for the driver circuits in addition to the switching elements. The TFTs of this sort for the driver circuits requires especially a high-speed response characteristic. Since poly-crystalline silicon (p-Si) films are remarkably higher in mobility than a-Si films, p-Si TFTs with active layers made of p-Si films are used for the driver-circuit integrated type liquid crystal display devices.
The p-Si films are usually formed in the following way. Firstly, an a-Si film is deposited on a glass substrate by a PECVD process. Next, dehydrogenation treatment is carried out for the a-Si film. In the dehydrogenation treatment, the glass substrate with the a-Si film deposited is received in a heating chamber in a low pressure atmosphere and is annealed at a lower temperature (e.g., 400.degree. C. through 500.degree. C.) than a heat-resistance temperature of the glass substrate in order to discharge hydrogen from the a-Si film. Finally, excimar laser beams are irradiated to melt and re-solidify the a-Si film so that the latter is changed into a p-Si film.
Reasons for such dehydrogenation treatment of the a-Si film are as follows. The a-Si film deposited on the substrate by the PECVD method contains a great quantity of hydrogen. When laser beams are irradiated to the a-Si film without the dehydrogenation treatment, hydrogen is discharged so abruptly from it that a phenomenon called "ablation" takes place to make defects in the p-Si film.
Conventionally, a heating chamber or furnace is exclusively and separately provided for the dehydrogenation treatment in addition to a PECVD system. The dehydrogenation treatment in such a heating chamber takes rather long time, e.g., a several hours including heating and cooling time of the glass substrate. Further, the glass substrate with the a-Si film should be washed before dehydrogenation treatment, as the case may be, because the substrate is exposed in the atmosphere after the PECVD process.
Because of the background stated above, the dehydrogenation treatment for the a-Si film in the production process of the p-Si film has been one of the critical factors to hinder its productivity.